Get the height used for a single Editor control such as a one-line EditorGUI.TextField or EditorGUI.Popup.
When creating your own multi-line controls, such as controls for custom classes with multiple fields, you can use EditorGUILayout.GetControlRect and pass in a height value that is a multiple of EditorGUIUtility.singleLineHeight. This is advisable over hardcoding specific pixel values. Additional resources: EditorGUILayout.GetControlRect.